Conventional optical projection lithography has been the standard silicon patterning technology for the past 20 years. It is an economical process due to its inherently high throughput, thereby providing a desirable low cost per part or die produced. A considerable infrastructure (including steppers, photomasks, resists, metrology, etc.) has been built up around this technology.
In this process, a photomask, or “reticle”, includes a semiconductor circuit layout pattern typically formed of opaque chrome, on a transparent glass (typically SiO2) substrate. A stepper includes a light source and optics that project light coming through the photomask to image the circuit pattern, typically with a 4× to 5× reduction factor, on a photoresist film formed on a wafer. The term “chrome” refers to an opaque masking material that is typically but not always comprised of chrome. The transmission of the opaque material may also vary such as in the case of an attenuating phase shift mask.
As the critical dimensions of integrated circuits continue to decrease, there is a need to pattern smaller and smaller features. Modern photolithographic systems often employ light in the imaging process which has a larger wavelength than the critical dimensions of the device features being formed on the integrated circuits. When critical dimensions are printed at less than or equal to the wavelength of light being used, the wave properties of the light become a dominant property of the lithography. In general, these wave properties are seen as being a limiting factor in lithography. There are, however, techniques for extending optical lithography beyond the range of conventional imaging.
One such technique, known as strong phase shift lithography, employs phase shift masks (PSM) to take advantage of the constructive and destructive properties of light to improve feature definition. Strong phase shift lithography is often used to pattern transistor gates in, for example, CMOS technologies, where a small, well-controlled gate length can yield considerable performance advantage.
One of the most common commercial implementations of phase shift mask technology is the double exposure method. In this method, the critical features are imaged using a phase shift mask, and the non-critical and trim features are imaged in a second exposure using a conventional chrome-on-glass mask, such as a trim mask. In the past, both the phase exposure and trim exposure were performed using a single photoresist.
More recently, a new process has been developed, referred to herein as two-pattern/two-etch (2p/2e) or “double patterning,” in which the phase exposure and trim exposure are each performed on separate photoresists. The patterns from each of the photoresists can be individually transferred to, for example, a hardmask. For example, a phase pattern may be formed in a first photoresist. The phase pattern can then be transferred to the hardmask using an etching technique. A trim pattern can then be formed in a second photoresist and the resulting photoresist pattern is then transferred to the hardmask using a second etching step. Subsequently, the hardmask pattern, having both the phase and trim patterns etched therein, is used to etch the wafer. In some processes, rather than employing a hardmask, the phase and trim patterns can be transferred directly to the wafer using the phase and trim photoresist patterns in two separate etch steps.
The 2p/2e processing allows for improvements in critical dimension control over single resist processing. However, the ever increasing densities of integrated circuit devices can make achieving the desired critical dimensions extremely difficult. Further refinements of the 2p/2e processing techniques are desired in order to achieve improved critical dimension control.